v2.2.0

This release modifies the gui slightly by moving the menu commands for subwindows from the parent window to the subwindow, as well as, the preferences interface. Also, the bug of all properties with an image show only the first properties image has been resolved (user needs to reload images, though). There have been minor tweaks with interoperability between subwindows, such as any fuel and maintenance modifications will immediately update the maintenance monitor. In addition, the code has been reorganized.